1. Field of the Invention
The present invention relates to mechanical clutches found in a ratchet tool, for example. More precisely, the present invention relates to improvements in a stepless ratchet one way clutch mechanism.
2. Prior Art And Related Information
Ratchet driving tools are well known, both in the form of ratchet wrenches and ratchet screwdrivers. Ratchet wrenches are distinct from ratchet screwdrivers by being of a different shape and importantly, by usually having a heavier construction. The purpose of either tool is similar in that oscillating rotation of a handle causes a drive shaft to progressively rotate. The rotation turns a screw or nut fastener.
Ratchet tools typically function by a toothed pawl engaging splines. The splines are spaced around the either the outside or the inside of a cylindrical surface. In the case of the former the cylindrical surface is part of the drive shaft. In the case of the latter the cylindrical surface is part of the tool body.
An improvement to ratchet tools is a stepless mechanism wherein a wedging action replaces the teeth. Two basic types of stepless actions are known. One design uses discrete rolling elements and transmits torque entirely by containing an efficient expanding force.
A second version is a cam wedge brake which combines a wedging action with friction to transmit torque. Since the desire in a driving tool is to transmit torque rather than to contain an expanding force, a device that combines wedging action with friction will be most effective.
A sprag clutch falls into the category of rolling elements. U.S. Pat. No. 4,162,000 shows a typical sprag clutch. Eccentric shaped sprag elements are held between an inner and an outer circular race. This type of design is quite common in machinery. A variation of the sprag clutch is shown in U.S. Pat. No. 5,482,144. In this design the sprag elements are wedged in two ways; first between the inner and outer races, and second within angled channels of each race. Another rolling element clutch is shown in U.S. Pat. No. 4,884,478. In this screwdriver design three round rollers fit between a cylindrical outer wall and an eccentric rotatable shaft. This differs from the sprag clutch in that the shaft is eccentric rather than the rolling elements. Designs similar to the '478 patent are the most common type mechanism for stepless ratchet hand tools. Either three or four rollers are typical. Another similar design is contained within a wrench handle. U.S. Pat. No. 4,669,339 is distinct from the '478 patent above only in certain details. Balls 56 of the '339 patent provide the direction selecting bias in the same manner shown by ball 48 in FIG. 6 of the '478 patent.
Similar roller type clutches have been known for more than 100 years. U.S. Pat. No. 592,213, from 1897, shows a roller clutch with direction selecting means similar to the above '478 and '339 patents.
To transmit high torques a rolling element clutch must have large numbers of rollers as in a sprag clutch used for machinery, or it must be large in size. The simpler clutch using fewer rollers still requires precision tolerances to achieve limited torque.
Various one way clutches of the friction style are known. For example, U.S. Pat. No. 2,735,324 discloses a stepless ratchet wrench using a wedging brake action. A brake element is forced into a gap between the rotating shaft and housing body. Empirical testing has indicated this design is not effective.
U.S. Pat. No. 2,766,648 shows an improved wedging method. The rotating shaft features a "V" shaped outer circumference. A corresponding V-contoured wedge element presses the shaft in direct proportion to a torque on the handle. The "V" shape functions similarly to an automotive "V" belt, wherein the contact friction is amplified by wedging in the "V." In this case a double wedging action occurs, first from the leverage from the handle, and second from the wedging within the "V."
U.S. Pat. Nos. 3,865,219 and 3,877,556 are similar to each other. A rotatable shaft includes an eccentric cam which forces shoe elements outward to press the inside wall of a housing. Both the '219 and '556 patents disclose embodiments where the shoe to housing interface comprise "V" contoured surfaces. Specifically surfaces 16" in FIG. 6 of the '556 patent, and elements B and P in FIG. 4 of the '219 patent are "V" contoured engagements.
The friction clutches of the '219 and '556 patents present limited suitability as reversible designs. In these two similar references the "shoes" each have a concave recess to fit a convex element of a cam or drive head. The requirement that a shoe concave portion precisely fit a cam convex feature limits the options for a reversible clutch. Specifically, the cam presses the shoe in only one area of the shoe. Therefore, for the cam to press the shoe from an opposing direction the shoe must translate to a substantially different location around the cam. Precisely this method of reversibility is proposed in FIG. 12 of the '219 patent.
In FIG. 12 the shoes are especially small, with adjacent empty space, to allow the large change of position of the shoes required to reverse direction. The shoe will thus be less sturdy than a non reversible design according to the '219 patent. This follows the concept that any empty space in the clutch head assembly is material lost which could otherwise be used for torque transmission, whether it is used in the form of a larger cam or a larger shoe.
A reversible roller type clutch similarly has much empty space to allow for the rollers to move about. Some reversible roller clutches show a direction selecting element filling space. FIG. 10 of U.S. Pat. No. 4,884,478 is a typical example. However, this is not a structural use of the space.
The present invention, in contrast, uses distinct and separated areas of contact between the drive head and the shoe or latch. By shifting the latch only very slightly around the drive head very different force directions may be applied to the latch. There remains very little non-structural space in the assembly.
As stated above, a roller clutch is actually too efficient. Specifically, a roller clutch will convert nearly 100 percent of a torque into an outward spreading force within the wrench housing. Under high torques the wrench head will expand outward causing the wrench action to flex or give. Further, unless the wrench is very large, the rollers will create indentations in the interior of the wrench head at high torques. Empirical observation has shown a 3/8 inch format roller clutch wrench made according to the design of U.S. Pat. No. 4,669,339 to suffer up to 10 degrees of flex at over 60 ft.-lb. of torque, and indentation damage near 150 ft.-lb of torque.
A 3/8 inch ratchet wrench should be stiff and durable to over 200 ft.-lb. of torque. A friction clutch is most effective since it is less efficient in converting torque into spreading force. Instead some of the torque is directly connected to the wrench body through friction. Since much of the spreading force is lost to friction the wrench head will expand less. Therefore, the action will be more solid and durable than with a roller clutch wrench.
Although some of the prior art friction drives may be effective in torque, they all suffer limitations of complexity and direction selecting ability. A sturdy reversible clutch is essential for a ratchet wrench hand tool.
A ratchet wrench in certain respects provides a highly abusive environment for a clutch device. The small device must survive high torques, dirty environments, impact, and non-axial bending. The last environmental condition highlights a deficiency of most prior art stepless ratchet tools. That is, especially in the case of wrenches, the handle is often pulled in and out in addition to around. This can create off axis forces in the drive head which tend to jam a stepless ratchet.
Furthermore, the conventional devices described above do not possess an effective means to hold the drive head securely on axis. The friction drives of the '219 and '556 patents teach no practical method of head retention for a ratchet wrench. Therefore, a need presently exists for a durable, low cost stepless ratchet tool mechanism with a simple means to switch direction.